A variety of permeability separatory devices in which the membrane takes the form of a large number of fine hollow fibers is known. In another type of application, hollow fibers composed of materials capable of transporting alkali metal cations and resistant to the molten metal, and to molten alkali metal polysulfides, have found use as electrolyte-separators in high temperature alkali metal-sulfur battery cells (see U.S. Pat. Nos. 3,476,602; 3,765,944 and 3,791,868, for example).
Common to both permeability separatory devices and battery cells of the foregoing types is the use of at least one "tubesheet" or relatively thin wall member which is sealingly engaged with the fiber exteriors and functions to separate different bodies of fluid in contact with the interior and exterior surfaces of the fibers (and to fix the position of the fibers within the device or cell).
Hollow fibers, i.e., thin-walled, hairlike tubules, are relatively fragile, particularly when composed of materials, such as glasses or ceramics, which are suitable for use in high temperature batteries. Consequently, it is quite difficult to avoid breaking at least several fibers when fabricating tubesheet-fiber assemblies. Even though only a few fibers out of a million may be broken, the resulting assembly will generally be useless for its intended purpose. This is particularly so in applications such as blood dialysis or high temperature batteries.
Several methods are known for salvaging tubesheet-hollow fiber assemblies made from resinous materials, but such methods are either inoperable or impractical when the assembly is composed of materials suitable for use in batteries operating at temperatures of 300.degree. C. or more and containing molten alkali metals and corresponding polysulfides (or halides).
Exemplary of known salvage methods are those disclosed in U.S. Pat. Nos. 3,499,062 and 3,968,192.
The U.S. Pat. No. 3,499,062 discloses (at columns 23 and 24) a method of repairing leaks in a fluid separation apparatus comprising a plurality of open-ended, generally parallel hollow fibers, potted at each end in "wall members" or tubesheets; and a tubular casing sealed to the peripheral surfaces of the tubesheets. Openings in the outer surface of a given tubesheet which connect to a leak through the tubesheet itself or to a leaking fiber are closed by forming a pool of a curable resinous liquid on the tubesheet surface, drawing enough of it into the leak connected opening to close off the leak or leaking fiber, removing the excess liquid and solidifying the remainder in place. (If the leak is in a fiber, the terminous of the fiber lumen in each of the two tubesheets must be so closed.)
In this method, flow of the resinous liquid into fibers which are not leaking is prevented by maintaining the liquid under a pressure which is greater than that exterior to the fibers but less than the pressure applied to the fiber lumens at their uncovered ends.
The U.S. Pat. No. 3,968,192 discloses an alternate method of salvaging leaky tubesheet-hollow fiber assemblies of the above-described type. The fiber and tubesheet materials are either thermoplastic or are heat-degradable to same, and the leak-connected openings are first located and then closed by localized, temporary application of heat and mechanical pressure to the materials in their immediate vicinity.
Neither of the foregoing methods is applicable to tubesheet-fiber assemblies which must function in corrosive, high temperature environments. In order to effectively plug any broken fibers in such assemblies, the plugging material, or "sealant", must meet several requirements which cannot be met by resinous materials. That is, the sealant not only must be fluid enough to be forced into the broken fibers (and any open channels through the tubesheet) under practicable operating conditions but also must be convertible, in situ and at temperatures below the distortion temperatures of the assembly materials, to a rigid, non-porous solid which is bonded to the fiber or channel walls, has a coefficient of expansion which adequately matches that of the fiber material and which will endure in the environment and at the temperatures it is exposed to during operation of the device.
Also, the technique used to prevent sealing off of good fibers in the process of the U.S. Pat. No. 3,499,062 cannot be used with fibers having closed ends. Although entry of the sealant into good fibers will be resisted by compression of the air in those fibers, enough sealant can still enter to effectively plug them.
A method of plugging defective fibers which is suitable for fiber/tubesheet assemblies to be used in high teperature batteries (and for remedying leaks in hollow fiber type permeability separatory devices as well) is disclosed in a co-pending application, Ser. No. 835,567, filed Sept. 22, 1977 in the name of W. E. Brown and F. Y. Tsang as inventors and entitled "Selective Plugging of Broken Fibers in Tubesheet-Hollow Fiber Assemblies". In this method, the open ends of the fibers protrude above the tubesheet and are immersed in a fluid, curable sealant. The broken fibers imbibe the sealant, by capillary attraction, to a level below the tubesheet surface. The sealant is not comparably imbibed by the good fibers, due to the resistance to compression of the gas they contain. The imbibed sealant is cured in place and the protruding fiber ends cut off flush with the tubesheet surface, thus re-opening the good fibers but leaving the bad fibers plugged.
The foregoing method has the disadvantage that, because the protruding fiber ends must be cut off after the sealing operation, it cannot be utilized if the fiber/tubesheet assembly has already been incorporated in a casing. Since fiber damage can occur during encasement of the assembly, a sealing method which avoids this disadvantage is highly to be desired. Also, the protruding fiber ends constitute something of a hindrance in handling assemblies which comprise them.